Problem: A circle has a circumference of $18\pi$. It has an arc of length $9\pi$. What is the central angle of the arc, in degrees? ${18\pi}$ ${180^\circ}$ $\color{#DF0030}{9\pi}$
Explanation: The ratio between the arc's central angle $\theta$ and $360^\circ$ is equal to the ratio between the arc length $s$ and the circle's circumference $c$ $\dfrac{\theta}{360 ^ \circ} = \dfrac{s}{c}$ $\dfrac{\theta}{360 ^ \circ} = 9\pi \div 18\pi$ $\dfrac{\theta}{360 ^ \circ} = \dfrac{1}{2}$ $\theta = \dfrac{1}{2} \times 360 ^ \circ$ $\theta = 180^\circ$